Hydraulic motor and pump system

ABSTRACT

The specification disclosed a hydraulic system for driving automobiles in which a separate fluid flow system is provided for the front and rear wheels respectively. Each of the drives for the front wheels and for the rear wheels includes three variable displacement pumps, one for pumping fluid and the other two each acting as a motor for one wheel. Both fluid driving pumps are driven by an internal-combustion engine. The hydraulic pumps and hydraulic motors all have counterbalanced vanes which minimize friction. A ratio control valve for each pump and each motor controls the torque to speed ratio at the vehicle wheels. A pressure equalizing valve equalizes pressure between the front and rear fluid flow systems. A bypass valve automatically shortcircuits the flow of fluid through a flow system when the vehicle&#39;&#39;s engine is idling.

HYDRAULIC MOTOR AND PUMP SYSTEM Inventor: Alan K. Caldwell, GrandRapids,

Mich. I

[73] Assignee: Rewop Company, Kent County,

Mich.

[22] Filed: Dec. 28, 1970 [21] Appl. No.: 101,447

[52] US. Cl. ..180/66 R, 60/19, 60/53 C, l37/493.7, 137/4938, 137/4939,418/31,

, 418/151 [51] Int. Cl. ..B60k 17/10 [58] Fieldot Search ..l80/66, 6.48;60/19, 53 C; 418/31, 16, 259, 151; 137/4939, 493.8, 493, 493.7

[56] References Cited UNITED STATES PATENTS 2,418,634 4/1947I-Iallerstrom ..l37/493.9 X 2,518,578 8/1950 Tomlinson ...l80/66 R UX2,547,578 4/1951 Holmes ..l80/66 R 2,598,538 5/1952 Haynes ..l80/66 R X3,256,950 6/1966 De Bias: ..l80/66 R X 3,422,917 1/1969 Guinot ..l80/66R 1 Apr. 3, 1973 FOREIGN PATENTS OR APPLICATIONS 2,009,975 10/1970Germany 1 80/66 R 858 1904 Great Britain ..418/151 576,420

[57] ABSTRACT The specification disclosed a hydraulic system for drivingautomobiles in which a separate fluid flow system is provided for thefront and rear wheels respectively. Each of the drives for the frontwheels and for the rear wheels includes three variable displacementpumps, one for pumping fluid and the other two each acting as a motorfor one wheel; Both fluid driving pumps are driven by aninternal-combustion engine. The hydraulic pumps and hydraulic motors allhave counterbalanced vanes which minimize friction. A ratio controlvalve for each pump and each motor controls the torque to speed ratio atthe vehicle wheels. A pressure equalizing valve equalizes pressurebetween the front and rear fluid flow systems. A

bypass valve automatically short-circuits the flow of fluid through aflow system when the vehicle's engine is idling.

14 Claims, 10 DrawingFigures 4/ 1946 Great Britain ..l80/66 R PATENTEDAPR 3 I975 SHEET 1 0F 4 FIG.

I mu NE w D m 1; K

r N w L: a

a Y B 2 3 m 0 3 o N 4 o 3 2 3 2 m, w 0 3 ATTORNEYS PATENTEDAPR 3 I975FIG. 5

SHEET 2 [IF 4 I NV E NTO R ALAN K. CALDWELL AT FORNEYS PATENTEDAPR 3I975 SHEET 3 BF 4 FIG.G

INVENTOR ALAN K./CALDWELL BY 12 ATTORNEYS PATENTEDAPR 3 I975 SHEET u 0F4 IGI FIG.B

l NV E NTO R ALAN K. CALDWELL ATTORNEYS BACKGROUND OF THE INVENTION Thepresent invention relates to hydraulic systems. In particular, itrelates to hydraulic systems for driving wheeled vehicles. While suchsystems are contemplated in the prior art, none of them have achievedsubstantial application as would be effected if such systems wereincorporated into automobiles.

Some prior art systems have no type of automatic variable transmission.Accordingly, the wheels rotate only in direct proportion to the rate ofrotation of the engine. If the engine is of the intemal-combustion type,it cannot be operated in its optimum range, and polluting emissions willresult. I

While some systems do employ variable displace ment, vane-type pumps,the pumps used suffer drawbacks. The friction of the rotating vanesagainst the vane tracks creates considerable wear and tear. Because thevanes are free to move outwardly against the vane tracks, this wear andtear increases as the rate of rotation of the pump increases.

Other hydraulic systems suffer the drawback that if a leak were todevelop, the vehicle would be stranded. While conventional powertransmission systems are also subject to possible breakdown, arevolutionary new type of drive system must be almost foolproof in orderto overcome consumer suspicion. Thus, the possibility of being strandeddue to fluidleakage is substantially eliminated. i i 3 Yet anotherdrawback is that in most systems, if not all, a clutch must be used inorder to stop the car without turning off the engine. Thus, those wholike automatic transmissions will -be discouraged from purchasingvehicles with such systems.

SUMMARY OF THE INVENTION The present invention contemplates a number ofimprovements over prior art hydraulic drive systems which substantiallyincrease the applicability of fluid drive systems. This may well resultin far reaching social benefits, since hydraulically driven vehiclesmade in accordance with this invention canbe operated in conjunctionwithinternal-combustion engines and yet can reduce the pollution emission ofsuch engines because the engine can be operated more efficiently.

In accordance with the present invention, means are provided for drivinga variable displacement, vane-type hydraulic pump. A variabledisplacement hydraulic motor, for at least one wheel, is then driven bythe pump. Means are provided for controlling the displace- V ment of thepump in direct proportion to the rate of rotation thereof and means areprovided for controlling the displacement of the motor in inverseproportion to the rate of rotation thereof. In this manner, the torqueto speed ratioof the system is greater at lower speeds than at higherspeeds and the means for driving the pump can be operated in a narrowerr.p.m. range than would otherwise be the case. Furthermore, where theprimary driving means is an intemal-combustion engine, it can beoperated almost exclusively within its optimum r.p.m. range of rotation,a range which is narrower than that required to drive a vehicle usingexisting mechanical transmission means.

In another aspect of the invention, it is contemplated that the variabledisplacement pumps and motors for use in this system includecounterweights mounted in the rotor of the pump or motor. These areoperably connected to the vanes and the vanes are radially slidablewithin the rotor. These counterweights act to offset the centrifiigalforce acting on the vanes and thereby minimize the friction between thevanes and the vane tracks of the pump or motor.

It is also contemplated that a separate fluid drive system be providedfor the front and rear wheels respectively with both systems beinginterconnected through a means for equalizing the pressure between thesystems. If one system breaks down, the other can still be used. The useof two separate systems is made possible by the pressure equalizationmeans since it allows the front wheels to rotate more rapidly than therear wheels as the car turns a corner. This, of course, is a necessityin order to minimize tire wear. Further, the use of two separate systemsallows one to economize by shutting one system down where the power oftwo would not be necessary.

Finally, the invention contemplates a system for short-circuiting themotors whereby fluid being pumped through the pump can be returnedthereto through a bypass or dump line. The bypass line is opened inresponse to the opening of a first and second valve and is closed inresponse to the closing of the first valve. Means are provided foropening the first valve in response to the release of the engineaccelerator and for closing the first valve in response to thedepression of the engine accelerator. Means are provided for opening thesecond valve below a preselected engine speed.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantagesof this invention will be seen by reference to the written specificationand appended drawings wherein:

FIG. 1 is a schematic view of the hydraulic drive system;

, FIG. 2 is a cross-sectional view of the variable displacementhydraulic pump or motor;

FIG. 3 is a sectional view of the pump or motor taken along planeIII-III of FIG. 2;

FIG. 4 is a cross-sectional view of the ratio control valve;

FIG. 5 is a cross section of the ratio control valve taken along planeV---\/ of FIG. 4; 7

FIG. 6 is a schematic of the control elements for operating the ratiocontrol valve;

FIG. 7 is a partially cutaway, sectional view showin the relationshipbetween the variable displacement pump or motor and the control elementsfor the ratio control valve;

FIG. 8 is a cross-sectional view of the pressure equalizing valve;

FIG. 9 is a cross-sectional view taken along plane IX-IX of FIG. 8; and

FIG. 10 is a schematic view ofv the bypass valve for the fluid flowsystems.

PREFERRED EMBODIMENT In the preferred embodiment, the invention includesan internal-combustion engine 10 which drives a pair of hydraulic pumps20a. The pumps 20a are each associated with an independent fluid. flowsystem, one

system A which drives the front wheels 11 and another system B whichdrives the rear wheels 11 (FIG. 1). In each fluid flow system, the pump20a drives a hydraulic motor 20b associated with each wheel 11. Thehydraulic motors 20b are identical to the hydraulic pumps 20a.

The pumps 20a and motors 20b are variable displacement types. The amountof fluid being pumped, or

passing through, can be varied by moving piston 23 either up or down(FIG. 2). A ratio control valve pump 60 (FIGS. 6 and 7) supplies a smallamount of fluid under pressure which can be directed by a control valve40 (FIG. 4) to the cylinder 22 of piston 23 (FIG. 6). A ratio controlvalve 40 and a ratio control bypass valve 90, for each pump 20a and eachmotor 20b, act in conjunction to control the torque to speed ratio ofthe system. They in turn are controlled by ratio control valve slinger70, which in response to acceleration and deceleration, regulates ratiocontrol valve 40 and ratio control bypass valve 90 to increase ordecrease fluid pressure at cylinder 22 of piston 23. Consequently, thewheels 11 operate at a higher torque to speed ratio at lower speeds forpurposes of acceleration and at a lower torque to speed ratio at higherspeeds.

A pressure equalizing valve 100 is-provided between the front and rearfluid flow systems in order to equalize the pressure therebetween (FIG.1). A bypass valve 120 is provided such that the motors 20b of eachfluid flow system can be short-circuited when the engine is at idlingspeed. As is customary in such hydraulic drive systems, a fluid flowreservoir 140 and a four-way control valve 160 are provided for eachfluid flow system A or B. For purposes of discussion, the specificationwill be divided according to the basic elements outlined above with theover-all operation of the system being described last.

7 PUMPS and MOTORS The pumps a and the'motors 20b are identical inconstruction and accordingly, a description of a pump 20a as shown inFIG. 2 will be sufficient. Pump 20a is a variable displacement, rotatingvane-type of pump having a casing 21 and a piston chamber 22. Casing 21is itment. In the case of pump 20a the rotation of rotor 26 causes thevanes 27 to draw fluid through an inlet 28 and force it'outwardlythrough an outlet 29 in casing 21. Outlet 29 is in flow-communicationwith line 161 and inlet 28 is in flow communication with line 162a (FIG.1). In the case of a motor 20b, the flow of fluid against vanes 27-causes rotor 26 to rotate. In motor 20b, inlet 28 is in flowcommunication with line 165 while outlet 29 is in flow communicationwith line 166.

The vanes 27 ride on an upper vane track 24 and a lower vane track 25.Lower vane track is stationary, but upper vane track 24 is mounted on apiston 23 which controls the relative position of vane track 24 withinthe interior of casing 21. Upper vane track 24 includes a slot 24a (FIG.3) which facilitates the flow of fluid into contact with the vanes 27.Lower vane track 25 includes a projecting end 25a which mates with slot240 such that upper track 24 is slidably guided thereby, in its verticalmovements within casing 21. It is important to note that piston 23 is aswide as casing 21 and therefore that fluid is unable to pass throughpump 200 except through the slots 24a. In other words, fluid is unableto flow up over the top of upper vane track 24 around piston 23.

Also mounted within rotor 26 are a plurality of counterweights 30, onefor each vane 27 (FIG. 2) These are designed to offset some of thecentrifugal force being exerted on vanes 27 during rotation of rotor 26.Wear of vane traclcs 24 and 25 is thereby minimized. Counterweights 30are mounted on the ends of pivoting arms 32, within hollow chamber 31 inrotor 26, such that counterweights 30 are free for swinging movementwithin motor 26. Counterweight 30 is operably connected to vane 27,since pivot arm 32 extends into a center of rotor 26 and bias vane 27radially outwardly towards engagement with either upper track 24 orlower vane track 25.

The relative amount of displacement of pump 20a is controlled by therelative position of uppervane track 24, which in turn is controlled bythe position of piston 23. A small opening 22a is provided in the top ofpiston chamber 22' whereby fluid can be fed under pressure to piston 23and can thereby control the relative displacement of pump 20a. Whenupper vane track 24 is in its uppermost position, pump 20a pumps a highvolume of fluid- When upper vane track 24 is in its lower position,

pump 20a pumps much less fluid, but pumps it at a greater rate of speed.Similarly in a motor 20b, the rotor 26 is turned most rapidly when uppervane track 24 is in its lowest position. However, more power isgenerated for axle 34 on rotor 26 when the upper vane track 24 of amotor 20b is in its uppermost position.

RATIO CONTROL VALVE As pointed out above, a ratio control valve 40 isprovided for each motor 20b and for each pump 20a.

These valves control the torque to speed ratio of the drive system. Whenacceleration is desired, greater torque is required. It can be achievedby increasing the The ratio control valve 40 (see FIG. 4) includes aninner chamber 41 within which is a rotatably mounted, rotating valveelement 42. It comprises a donut-like ring 42a and a narrower centralweb portion 42b (FIGS. 4 and 5). It is rotatably mounted on a centralaxle 43. It can be rotated in a clockwise or counterclockwise directionby means of controls which will be discussed infra. An intermediatechamber 44 arches around inner chamber 41 and there is a large interfaceopening 45 between the inner chamber 41 and intermediatechamber 44.Within intermediate chamber 44 there is a sliding valve element 46. Ascan be seen by reference to FIG. 5, sliding element 46 and secondchamber 42 are of closely conforming, circular cross sections. An outerchamber 47 curves around intermediate chamber 44 in a manner similar tothat in which intermediate chamber 44 curves around inner chamber 41.There is a large opening 48 between intermediate chamber 44 and outerchamber 47.

Inlet 49 is provided into the interior of inner chamber 41 (FIG. 5) andan outlet 50 is provided from outer chamber 47 (FIGS. 4 and 5). Flowcommunication can be effected between inlet 49 and outlet 56 throughpassages 51 or 52 in rotating valve element 42 and passage 53 in slidingvalve element 46. it should be noted that the fluid can flow in eitherdirection, i.e. from inlet 49 to outlet 50 or vice versa, depending onwhether there is a positive or negative pressure differential betweenthe two. This pressure differential is determined by the controls forratio control valve 40 to be discussed infra.

Within rotating valve element 42, there are spaced passages 51 and 52opening at oneend in web 42b. in this manner, these passages are in flowcommunication with inlet '49 as is indicated in FIG. 5. These extendfrom the interior of inner chamber 41, through ring 7 42b, and out tothe opening 45 between inner chamber 41 and intermediate chamber 44.There is a similar passage 53 in sliding valve element 46 which extendsfrom the opening 48 between intermediate chamber 44 and outer chamber 47to the opening 45 between inner chamber 41 and intermediate chamber 44.It should be noted that there is a shoulder 46a on valve element 46which abuts snugly against rotating valve 42. Passage 53 extends throughthis shoulder portion 46a and when valve element 46 is in its normalposition, the opening of passage 53 is oriented between the openings ofpassages 51 and 52. Thus, it is normally blocked because of the abutmentof shoulder portion 46a against rotating valve 42.

Valve element 46 is biased towards counterclockwise sliding movement bymeans of a spring 54 which is mounted within intermediate chamber 44. Atthe other end of valve element 46, there is a small leakback passage 55between outer chamber 47 and intermediate chamber 44. This is controlledby a metering screw 56. When the controls for ratio control valve 46cause rotating valve element 42 to be rotated in a clockwise directionuntil passage 51 and passage 53 align, and when the controls cause fluidpressure to be increased at inlet 49 with respect to outlet 50, fluidwill flow through passage 51, passage 53, into outer chamber 47 and outthrough outlet 50. However, because of increased pressure in outerchamber 47, some fluid will flow through leak-back passage 55, intointermediate chamber 44 at one end of sliding valve element 46 and willthereby force sliding valve element 46 to move in a clockwise direction.Thus, passage 53 will gradually be brought out of alignment with passage51. This will cause the flow of fluid to stop.

if rotating valve element 42 is rotated in a counterclockwise directionuntil passage 52 and passage 53 align, and if pressure is decreased atinlet 49 with respect to outlet 50, fluid will flow from outlet 50,through passage 53, passage 52 and out through inlet 49. The resultingdecrease in pressure inouter chamber 47 will cause fluid to flow out ofintermediate chamber 44 through leak-back passage 55. This will causespring element 54 to move sliding valve 46 in a counterclockwisedirection and will cause passage 53 to move out of flignment withpassage 52. In this manner, a small amount of fluid can be caused toflow from inlet 49 to outlet 50 or vice versa for a short period oftime, the length of time depending on the setting of metering screw 56.

CONTROLS FOR RATIO CONTROL VALVE in using ratio control valve 40 tocontrol a pump 20a or motor 20b, outlet 50 is connected by means of afluid flow line to opening 22a in piston chamber 22 of pump 20aor'rnotor 2012 (FIG. 6). The relative rotation of rotating valve element42 and the relative pressure at inlet 49 with respect to outlet 50 arethen controlled by a ratio control valve pump 60, a ratio control valveoil slinger 76 and a ratio control bypass valve 90 (FIG. 6).

Pump 64) comprises a small sliding piston 61 which is biased by a spring63 into engagement with a cam 62 protruding from the surface of shaft 34of pump 20a. Pump is physically contained within a second casing 2101which is contiguous with casing 21 of pump 20a (H6. '7). A small passage66 is provided between the interior of casing 21, i.e. the interior ofpump 20a, to the pumping chamber of piston 61. A pair of check valves 64are provided on either side of the pumping chamber of piston 61 suchthat piston 61 acts to pump a very small amount of fluid out of theinterior of casing 21 and then through a line 65 which is secured at itsopposite end to inlet 49 of ratio control valve 40. 7

Line 65 also branches off to ratio control bypass valve (FlG. 6). lit isthe operation of bypass valve 90 which determines whether or not thepressure applied at inlet 49 is positive or negative with respect tooutlet 50. 1f bypass valve 90 is opened, then the pressure differentialis negative since fluid flows freely through line 93 back to pump 200.if it is closed, the pressure differential is positive.

The opening and closing of bypass valve 90, as well as the rotation ofrotating valve element 42 are controlled by ratio control valve oilslinger (FIGS. 6 and 7) which senses increases and decreases inrotational speed of the engine or of the wheels. Oil slinger 70 includesa plurality of slinger vanes 71 which are mounted on shaft 34 withincasing 21a. For pumps 20a, the rate of rotation of shaft 34 is afunction of engine speed. For motors 20b, the rate of rotation of shaft34 is a function of vehicle speed. During the operation of pump 20athere will be at least some oil within the interior of casing 21a. Bymeans of centrifugal force which is variable in accordance with thespeed of rotation of shaft 34, slinger vanes 71 act to sling this oiloutwardly away from shaft 34. A sensing ring 72 is rotatably mountedabout the circumference of the slinger vanes 71. It includes a webportion 72a which extends down to shaft 34 upon which sensing ring 72 isrotatably mounted. The web portion 72a holds sensing ring 72 in positionabout the circumference of slinger vanes 71 (FIG. 7). Extendingdownwardly from the interior of sensing ring 72 are a plurality of smalltabs 73 which catch the oil being slung by slinger vanes 71 and therebycause sensing ring 72 to rotate. Sensing ring 72 is biased against suchrotation by means of a spring 76.

Slinger 70 controls ratio control valve 40 by means of a control rod 58extending from a small slot 77 in sensing ring 72 to control arm 57which is rigidly secured to the axle 43 of rotating valve element 42(FIG. 6). Thus, it can be seen that a clockwise rotation of sensing ring72, which will occur when the rate of rotation of shaft 34 on pump aincreases will cause a counterclockwise rotation of rotating valveelement 42. Slot 77 allows for slight fluctuation in sensing ring 72without affecting rotating valve element 42. When the rate of rotationof shaft 34 on pump 20a decreases, sensing ring 72 will be rotatedcounterclockwise by spring 76, and rotating valve element 42 will berotated clockwise by rod 58.

Ratio control bypass valve 90 includes a control arm 91 and a controlrod 92 which extends therefrom to the top of a quadrant gear 75.Quadrant gear 75' is rotatably mounted in position above sensing ring 72and the top portion of sensing ring 72 is provided with gear teeth 74such that rotation of sensing ring 72 in a clockwise direction resultsin a corresponding counterclockwise rotation of quadrant gear 75 andresults in the opening of bypass valve 90, to line 93 which empties backinto pump 20a (FIG. 6).

There are two essential differences between the control arrangements fora ratio control valve 40 when it is used to control a pump 200 asopposed to when it is used to control a motor 20b. When the ratiocontrol valve 40 is being used to control a pump 20a, it is connected tosensing ring 72, as indicated above, such that when sensing ring 72rotates in an increasing speed direction with an increase in enginespeed and correspondingly in pump rotational speed, rotating valveelement 42 rotates in a counterclockwise direction as viewed in FIG. 4.Similarly, control rod 92 is connected to control arm 91 of bypass valve90 such that rotation of sensing ring 72 in the increasing speeddirection results in an opening of bypass valve 90 and a decrease inpressure at inlet 49. This means that as the rotational speed of pump20a increases due to an increase in engine speed, fluid will graduallyflow from the piston chamber 22 of pump 20a and accordingly, the volumedisplacement of pump 20a will increase as the engine speed increases.Contrariwise, the volume displacement of pump 20a will decrease asengine speed decreases. When the ratio control valve 40 is used tocontrol a motor 20b, the arrangement of the control rods 58 and 92 isjust the opposite such that rotating valve element 42 rotates clockwise(as viewed in FIG. 4) when sensing ring 72 rotates in an increasingspeed direction and bypass valve 90 closes when sensing ring 72 rotatesin an increasing speed direction. In this manner, the displacement of amotor 20b decreases with increasing vehicle speed and increases withdecreasing vehicle speed.

The second difference between the control arrangements for a ratiocontrol valve when it is used to control a pump 20a, rather than a motor20b, is that spring 76 is operably connected to the accelerator forengine 10. Thus, depression of the accelerator results in a rotation ofsensing ring 72 in a counterclockwise direction, thereby causingrotating valve element 42 to rotate clockwise and thereby causing ratiocontrol bypass valve 90 to close. Thus, depression of the acceleratorimmediately sets the displacement of the pumps 20a at their minimumvalues. The spring 76 in a system associated with a motor 20b couldsimilarly be connected to the accelerator such that depression of theaccelerator would result in an immediate shift in displacement of motor20 to its maximum value. However, it is felt that this will not benecessary and would merely result in changes in velocity which would betoo abrupt upon the depression or release of the accelerator for engine10.

PRESSURE EQUALIZING VALVE Pressure equalizing valve 100 is positioned inflow communication between the front and rear hydraulic drive systems Aand B (FIG. I). It is cylindrical in shape and includes an inner chamber101 (FIG. 8) which extends generally the length thereof. A spool 102 ismounted for slidable movement within inner chamber 101 (FIGS. 8 and 9).Spool 102 includes two identical end blocking portions 103 and a centralno fluid can flow therethrough. Spool 102 is spring biased to agenerally central position within inner chamber 101 by means of a spring106 at each end thereof and extending between an end wall of innerchamber 101 and an end blocking portion 103 of spool 102. In thismanner, spool 102 is normally biased to a generally central positionwithin inner chamber 101.

There is an opening into each end of inner chamber 101. One opening 105is connected to the front fluid drive system A and the other isconnected to the rear fluid drive system B. In this manner, fluid isfree to communicate to the interior of inner chamber 101 from each ofthe fluid flow systems A and B. Inner chamber 101 is surrounded by anouter chamber 107. Inner chamber 101 and outer chamber 107 are capableof communicating through the passages 108 which extend laterally throughthe wall separating inner chamber 101 from outer chamber 107. However,the passages 108 are positioned such that they are blocked by spool ends103 when spool 102 is in its normal, central position. There must be atleast one passage 108 at each spool end 103. However, there may be moreand in the preferred embodiment there are four such passages 108 at eachspool end 103. It is important that these passages 108 be normallyclosed such that normally, there is no flow communication between innerchamber 101 and outer chamber 107.

In the event that the pressure in one of the fluid flow systems becomesgreater than the pressure in the other,

spool 102 will shift slightly to the left or right. Assuming an increasein pressure in front fluid flow system A, the passages 108 will beopened and fluid will be able to flow from sub-chamber a, out throughpassages 108, into outer chamber 107 and back through the passages 108at the other end into sub-chamber c. From thence fluid will flow throughpassages 109, into sub-chambers cand through left end opening 105 intorear fluid flow system B. Conversely, an increase in pressure in rearfluid flow system B will cause spool 102 to shift to the right. Fluidwill flow from sub-chamber d, to outer chamber 107, to sub-chamber b, tosub-chamber a, and into front fluid flow system A. When the pressuresbetween the front and rear systems A and B again equalize, spool 102will return to its normal position as shown in FIG. 8.

Each biasing spring 106 is positioned directly over each opening 105.Accordingly, a short stand pipe 110 is provided which extends a shortdistance into the interior of inner chamber 101 at each end thereof. Thelength of stand pipe 110 is slightly greater than the length of spring106 when it is fully compressed. This insures that a complete seal willbe effectuated between blocking end 103 of spool 102 and opening 105 inthe event that either of the front or rear fluid flow systems A or Bdepressurizes completely.

BYPASS VALVE There are times during the operation of a wheeled vehiclewhen a person will desire that the engine be idling and the vehicle bestationary. Accordingly, a bypass line 121 having a bypass valve 120thereon is provided directlyfrom the pump supply line 161 to reservoir140 (FIG. 1). Bypass line 121 acts to shortcircuit the entire fluid flowsystem such that no driving fluid is delivered from pump a to motors20b. Bypass valve 120 comprises a rotating valve member having a controlarm 122 thereon to effectuate its rotation (FIG. 10). Control arm 122 isoperably connected by a control rod 123 to a plunger 124. Plunger 124is'spring biased by means of spring 125 such that bypass valve 120 isnormally closed as is shown in FIG. 10. Plunger 124 is connected to line126 which is in flow communication with a solenoid valve 127 and a flyball governor valve 128, both connected in series on line 126. A vacuumline 129 extends from fly ball governor valve 128 to the engine manifoldvacuum. When both valves 127 and 128 are opened, vacuum is supplied toplunger 124, via lines 129 and 126, and tends to pull it against theaction of spring 125 and thereby open bypass valve 120. When solenoidvalve 127 closes, to block line 126, it opens to the atmosphere, therebyallowing spring 125 to again force plunger 124 to its normal positionand close bypass valve 120.

Solenoid valve 127 is operated by means of a switch part which reflectsthe rpm of the engine. This fly ball governor might be convenientlyattached to the distributor rotor. The snap-type fly ball governor isadjusted such that when the engine rpm decreases to a certain lever,valve 128 is opened. Above that preselected rpm, valve 128 will beclosed and below that preselected rpm valve 128 will be opened.

Thus, if a car is idling, the valve 128 will be open and solenoid valve127 will be open to the engine vacuum. Accordingly, bypass valve 120will be opened and the fluid flow systems will be short-circuited. Assoon as the accelerator is depressed, however, solenoid valve 127 willopen to the atmosphere and bypass valve 120 will be closed. Whenoperating at higher rates of speed, the release of the accelerator willcause solenoid valve 127 to open to the manifold vacuum. Valve 128 willstill be closed, however, and the motors 20b will have the brakingbenefit of fluid back pressure. As soon as the engine speed decreases toa predetermined level, the snap-type fly ball governor will eflectuatean opening of valve 128 and the manifold vacuum will act on plunger 124to open bypass valve 120.

FLUID FLOW CONTROL The vehicle direction is controlled by a conventionalfour-way control valve 160 (FIG. 1). There is one such valve 160 foreach fluid flow system A and B, and the flow arrangement for each fluidflow system A and B is identical. Fluid is pumped by pump 20a throughpump supply line 161 to four-way control valve 160. From here it flowseither through a first line 163 or a second line 164 to either a firstmain fluid line 165 or to a second main fluid line 166 respectively. Theinlets 28 are connected to lines 165 while the outlets 29 are connectedto lines 166. The particular path which the fluid will follow, i.e.,either through first line 163 or second line 164 depends upontherelative position of four-way control valve 160. Assuming the vehicle isto travel in a forward direction, the fluid will flow through first line163 into first main line 165. From thence it will flow to each of themotors 2012, through the motors 20b and out through second main line166. It will then return through second line 164, into four-way valve160 and out through pump return line 162. Pump return line 162 emptiesinto reservoir and line 162a draws fluid from reservoir 140 into pump20a. In the event that four-way valve is adjusted toits second position,fluid will flow through second line 164 and second main flow line 166and will return via main line and first line 163. Each main line, 165and 166 includes a shock absorber 167 at one end thereof to absorb thenumerous shocks which will be caused in the system for various reasons,such as the application. of brakes, sudden reversalof direction, etc. Inthe preferred embodiment, these shocks comprise small spring biasedplungers at each end of each main flow line.

on the accelerator peddle for engine 10. When the accelerator isdepressed, solenoid valve 127 is actuated to open to the atmosphere,thereby closing bypass valve 120 and causing fluid to be pumped frompump 20a to motors 20b. When the accelerator is released, solenoid valve127 opens to engine manifold vacuum line 129.

However, this in and of itself does not open bypass valve 120 sincevalve 128 is in series with solenoid valve 127. Valve 128 is controlledby a snap-type fly ball governor which is connected to a rotating'engineOPERATION With the operation of the above specific components now beingclearly understood, the over-all operation of the invention can beunderstood by conducting a series of hypothetical maneuvers. Beginningwith idling, the car will be accelerated, decelerated, turned around acorner, and brought to a stop. The operation of the various componentsduring these maneuvers will be explained.

The internal-combustion engine is initially activated and the car isrunning at an idling speed. Bypass valve 120 is open such that wheelmotors 20b are being shortcircuited. Solenoid valve 127 and valve 128(FIG. are open to the manifold vacuum. In order to initiateacceleration, the engine accelerator is depressed and solenoid valve 127is immediately switched to the position whereby it is opened to theatmosphere. This causes spring 125 to act on plunger 12A and therebyclose bypass valve 120. Fluid now begins to flow to wheel motors b. Thedepression of the accelerator also acts upon spring 76 to rotate sensingring 72 associated with each pump 20a in a counterclockwise direction.This causes rotating valve element 42 (FIG. 4) of each ratio controlvalve 40 associated with each pump 20a to rotate in a clockwisedirection until first passage 51 is aligned with third passage 53 insliding valve element 46. Simultaneously, bypass valve 90 is closed,thereby effectuating a positive pressure differential at inlet 49 withrespect to outlet 50. Fluid flows through passage 51 and passage 53 outthrough outlet 50 and into the piston chambers 22 of the pump 200 vialine 50a (FIG. 6). This causes piston 23 to move downwardly andeflectuates a decrease in the displacement of pumps 200. Upper vanetrack 24 is moved downwardly, forcing vanes 27 radially inwardly anddecreasing their pumping area. This causes pumps 20a to pump a smallervolume of fluid at a greater velocity. With the vehicle at rest, thedisplacement in the motors 20b is at its highest point, thus causing thefluid which is flowing from the pumps 200 at a very high velocity toexert a great deal of torque on the motors 20b and correspondingly onthewheels 1 1.

As the vehicle begins to accelerate, the slinger vanes 71 associatedwith all of the ratio control valves 40 begin to sling oil against thevanes 73 and thereby cause the sensing rings 72 to rotate in anincreasing speed direction against the relative bias of springs 76. Notethat this will be clockwise or counterclockwise for mo tors 20bdepending on whether they are on the left side or right side of the car.As set forth herein, the increasing speed direction for sensing rings 72on pumps 20a is clockwise. The rotation of sensing ring 72 in anincreasing speed direction has one effect on the ratio control valves 40associated with the pumps 20a and an opposite effect on the ratiocontrol valves 40 associated with the motors 20b.

With respect to the pump ratio control valves, this rotation of sensingring 72 causes rotating valve element 42 of valve 40 (FIG. 4) to rotatein a counterclockwise direction as viewed in FIG. 4 and causes ratiocontrol bypass valve 90 to open. Passage 52 of rotating valve element 42moves into alignment with passage 53 in sliding valve element 46. Theopening of ratio control bypass valve 90 elfectuates a negative pressuredifierential at ratio control valve inlet 49 with respect to outlet 50by allowing fluid to return directly to the casing of pump 20a. Thiscauses fluid to flow from piston chamber 22 of each pump 20a backthrough passage 53 and passage 52. Gradually, the decrease in pressurewithin outer chamber 47 causes fluid to leak from intermediate chamber44 through leak-back passage 55 and thereby allow spring 54 to act uponsliding valve element 46 pushing passage 53 out of alignment withpassage 52 and thereby cutting off the flow of fluid out of cylinder 22.

When the accelerator pedal is released, to initiate deceleration, thefirst reaction of sensing ring 72 associated with a pump 20a is torotate in a clockwise direction due to the interconnection of spring 76with the engine accelerator pedal. However, as the engine These rotatingvalve elements 42 will rotate in a clockwise direction and bypass valveswill close. This will result in the same effect as is created in theratio control valves 40 which are associated with the pumps 20a when theengine accelerator is initially depressed. Accordingly, therelativedisplacement of the motors 20b will now become less and the large volumeof fluid being pumped by pumps 20a will move through the motors 20b at avery rapid rate of speed, thus eflectuating a high rate of rotation ofwheels 11. When the vehicle begins to decelerate, the above process isrepeated in reverse due to the action of the springs 76 which normallytend to pull the sensing rings 72 in a decreasing speed direction.

When the vehicle is turned around a comer, the front wheels 11 will haveto rotate at a slightly faster speed than the rear wheels 11. In effect,a back pressure will be built up in the rear fluid flow system B.Assuming that the rear fluid flow system B is connected to the left sideof pressure equalizing valve as viewed in Flg. 8, the increase inpressure at the left end of inner chamber 101 will cause spool 102 toshift to the right. Fluid-will flow from sub-chamber d, into outerchamber 107 through the passages 108 at the left end of the valve andback into sub-chamber b through the passages 108 at the right end of thevalve. From thence they will flow through the axial passages 109 inspool end blocking portion 103 and out through the opening at the rightend of pressure equalizing valve 100. This influx of fluid into thefront fluid flow system A and out of the rear fluid flow system B willallow the rear wheels 11 to rotate more slowly than the front wheels 1 las the car is turned through a corner.

If a leak were to develop in the rear fluid flow system, then it wouldcompletely depressurize and spool 102 would shift completely to theleft. The left end 103 of spool 102 would abut the left end stand pipe110 and effectuate a seal at that point such that fluid would not beable to flow out of front fluid flow system A intothe depressurized rearfluid flow system B.

In order to stop the car, the brakes can be applied and the vehicle willbegin to decelerate. The shock created by the application of the brakeswill be somewhat absorbed by the shock absorbers 167 in the main fluidflow lines and 166. Once the foot has been removed from the accelerator,solenoid operated valve 127 will open to the manifold vacuum. However,valve 128 will remain closed until the car has decelerated such that theengine rpms reach a predetermined low rate. At this point, the snap-typefly ball governor will cause valve 128 to open and a vacuum will beapplied to plunger 124 which will then open bypass valve 120.

Throughout the vehicle operation, the pumps a and motors 20b areoperated with a minimum of friction on the vane tracks 24 and 25. Thisis due to the action of counterweights for each vane 27. As centrifugalforce increases on a vane 27 due to increased rate of rotation of rotor26, it is also increased on counterweight 30. Thus, the force of vane 27against vane tracks 24 and 25 is not substantially increased withincreased rotational speeds. Positive contact of vanes 27 with tracks 24and 25 is insured by springs and by the fact that the moment arm ofcounterweight 30 is slightly less than that of vane 27.

Thus, it can be seen that the present invention comprises a number offeatures which cooperate to provide a unique hydraulic drive system forwheeled vehicles. If one desired to use only one of the two fluid flowsystems, a simple switching mechanism can be used to effectuate hischoice. In effect, an automatic clutch is provided by the unique bypassvalve 120. Automatic power transmission is provided by the unique ratiocontrol valve and its depending control elements. Pres sure equalizingvalve 100 provides a unique four-wheel drive differential. Finally, theunique variable displacement pump provided suffers only a minimum ofwear because of the counterweighted vanes provided therefor.

It is understood that the above is merely a preferred embodiment of theinvention and that many changes and alternations can be made thereofwithout departing from its spirit and broader aspects.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

l. A hydraulic drive system for a wheeled vehicle comprising: ahydraulic pump having a means for varying its effective displacement;means for driving said hydraulic pump; a hydraulic motor for at leastone wheel, said motor having means for varying its effectivedisplacement and being driven by said pump; said means for varying theeffective displacement of said pump including control means forcontrolling the effective displacement of said pump in direct proportionto the rate of rotational speed thereof; said means for varying theeffective displacement of said motor including control means forcontrolling the effective displacement of said motor in inverseproportion to the rate of rotational speed thereof, whereby the torqueto speed ratio at said driven wheel is greater at lower vehicle speedsthan at higher vehicle speeds; each of said variable displacement pumpand motor including a piston for controlling the displacement thereof,and each of said pump controlling means and said motor controlling meanscomprising: a valve having first and second chambers therein; first andsecond valve elements movably mounted in said first and second chambersrespectively; a first opening in said first chamber being in flowcommunication with a source of fluid; a second opening in said secondchamber being in flow communication with the displacement piston of oneof said variable displacement pump and variable displacement motor; anintermediate opening between said first and second chambers, said valveelementsbeing in slidable, abutting engagement at said intermediateopening; first and second spaced passages through said first valveelement in flow communication with said first chamber opening andextending to said intermediate opening; a third passage in said secondvalve element in flow communication with said second chamber opening andextending to said intermediate opening; the opening of said thirdpassage at said intermediate opening being positioned between theopenings of said first and second passages at said intermediate openingwhereby it is normally closed because of said slidable, abuttingengagement of said first and second valve elements; means for increasingthe pressure at one of said first chamber opening and second chamberopening with respect to the other and for moving one of said first andsecond passages into alignment with said third passage to eflectuatefluid flow between said fluid source and said piston; means for movingsaid second valve element and correspondingly said third passage out ofalignment with one of said first and second passages and in the samedirection in which said first valve element was originally moved tothereby again discontinue the flow of fluid between said fluid sourceand said piston; said valve for said pump being adjusted such that fluidflows from said fluid source to said piston during vehicle decelerationand flows from said piston to said fluid source during vehicleacceleration; said valve for said motor being adjustedsuch that fluidflows from said piston to said fluid source during vehicle decelerationand from said fluid source to said piston during vehicle acceleration.

2. The system of claim 2 in which said means for moving said secondvalve element in the same direction in which said first valve elementwas originally moved comprises said second valve chamber being in fluidflow communication with said second opening at one end of said secondvalve element, whereby changes in relative pressure at said secondopening are gradually transmitted to said one end of said second valveelement in said second chamber; bias means being provided at the otherend of said second valve element for biasing it toward one direction ofmovement within said second chamber.

3. The system of claim 2 in which said means for moving said first valveelement and for effectuating a pressure differential between said firstand second chamber openings comprises: governor means for applying aforce to said first valve element which is proportional to the rate ofrotation of one of said pump and said motor; bias means being operablyassociated with said governor means for opposing said force applied tosaid governing means; pump means for said fluid source; bypass valvemeans associated with said fluid source and controlled by said governorfor opening and closing a return line to said fluid source pump wherebythe fluid pressure at said first chamber opening of said valve can beincreased or decreased depending on the relative position of said bypassvalve.

4. A hydraulic drive system for a wheeled vehicle comprising: ahydraulic pump having a means for varying its efl'ective displacement,means for driving said hydraulic pump; a hydraulic motor for at leastone wheel, said motor having means for varying its effectivedisplacement and being driven by said pump; said means for varying theeffective displacement of said pump including control means forcontrolling the effec tive displacement of said pump in directproportion to the rate of rotational speed thereof; said means forvarying the effective displacement of said motor including control meansfor controlling the effective displacement of said motor in inverseproportion to the rate of rotational speed thereof, whereby the torqueto speed ratio at said driven wheel is greater at lower vehicle speedsthan at higher vehicle speeds; a first fluid flow system for driving thefront wheels and a second fluid flow system for driving the rear wheelsof said vehicle, said first and second fluid flow systems eachcomprising at least one variable displacement pump and at least onevariable displacement motor; means interconnecting said first and secondfluid flow systems for equalizing pressure differentials which may arisetherebetween during the operation of said vehicle; said means forequalizing pressure comprising a pressure equalizing valve including afirst chamber and a spool mounted therein having two end portions, and acentral portion around which fluid in said first chamber cannot flow;said spool'being mounted for slidable movement in said first chamber; anopening into each end of said first chamber, being in flow communicationwith one of said two fluid flow systems; means biasing said spool towarda normal position in said first chamber; a second chamber in saidpressure equalizing valve; at least one passage between said first andsecond chambers being positioned at each spool end when said spool is inits normal position, such that said passages are normaliy blocked bysaid spool ends; said spool being slidable in said first chamber suchthat upon increase in pressure in one of said fluid flow systems, saidspool shifts away from said opening which is in flow communication withthe increased pressure system, toward the other opening in said firstchamber such that said passages between said first and second chambersare open; each end of said spool including passages extendingthere'through such that when said passages between said first and secondchambers are open, fluid can flow from the high pressure end of saidfirst chamber, through said second chamber, back into said first chamberin the space between said spool end and said spool central portion andthrough said passages in said spool end and through said opening in saidfirst chamber into the lower pressure system.

5. The system of claim 4 in which said spool is biased by a springpositioned at each of said ends, said openings into each end of saidfirst chamber comprising a stand pipe extending into said chamber for adistance greater than the length of said spring when it is fullycompressed such that a seal is effected by the action of said spool endpressing against the end of said stand pipe in the event that one ofsaid two fluid flow systems completely depressurizes.

6. A hydraulic drive system for a wheeled vehicle comprising: ahydraulic pump having a means for varying its effective displacement;means for driving said hydraulic pump; a hydraulic motor for at leastone wheel, said motor having means for varying its efiectivedisplacement and being driven by said pump; said means for varying theeffective displacement of said pump including control means forcontrolling the eflective displacement of said pump in direct proportionto the rate of rotational speed thereof; said means for varying theeffective displacement of said motor including control means forcontrolling the effective displacement of said motor in inverseproportion to the rate of rotational speed thereof, whereby the torqueto speed ratio at said driven wheel is greater at lower vehicle speedsthan at higher vehicle speeds; each said variable displacement pump andsaid variable displacement motor comprising: a vane track having amovable section whereby variable displacement is achieved; a rotorcarrying a plurality of vanes mounted for radially slidable movementtherein, a counterweight mounted in said rotor for each of said vanesand being operably connected to said vane for offsetting the centrifugalforce acting on said vane during rotation of said rotor.

7. The system of claim 6 in which said counterweight and said vane areconnected through a pivoted arm; the moment arm of said vane about thepivot of said pivoted arm being slightly greater than the moment arm ofsaid counterweight to held insure continuing contact between said vaneand said track.

8. The system of claim 7 in which said pivoted arm extends into a slotin said vane and fits loosely therein whereby said vane is free to moveradially within said rotor while said pivoted arm moves through an areabout its pivot point.

9. The system of claim 7 in which said counterweight and said vane areconnected through a pivoted arm;

bias means being operably connected to said arm for biasing said vane tomovement radially outwardly whereby continuing contact between said vaneand said track are insured.

10. The system of claim 6 in which said counterweight and said vane areconnected through a pivoted arm; bias means being operably connected tosaid arm for biasing said vane to movement radially outwardly wherebycontinuing contact between said vane and said track are insured.

11. A hydraulic drive system for a wheeled vehicle comprising: ahydraulic pump having a means for varying its efiective displacement;means for driving said hydraulic pump; a hydraulic motor for at leastone wheel, said motor having means for varying its effectivedisplacement and being driven by said pump; said means for varying theeffective displacement of said pump including control means forcontrolling the efiective displacement of said pump in direct proportionto the rate of rotational speed thereof; said means for varying theeffective displacement of said motor including control means forcontrolling the effective displacement of said motor in inverseproportion to the rate of rotational speed thereof, whereby the torqueto speed ratio at said driven wheel is greater at lower vehicle speedsthan at higher vehicle speeds; a return reservoir and a bypass line tosaid reservoir are provided whereby the fluid pumped by said pump can bereturned through said bypass line to said reservoir rather than beingcirculated to said motor; a bypass valve on said bypass line for openingand closing said bypass line; a vacuum line to a vacuum source; firstand second valve means positioned in series on said vacuum line wherebysaid vacuum line is open only when both said first and second valvemeans are open; means for opening said first valve when said engineaccelerator is released and for closing said first valve when saidengine accelerator-is depressed; means for opening said second valvebelow a preselected engine speed and for the opening of said first andsecond valves causes a vacuum to act on said control piston, againstsaid bias ing means, and thereby open said bypass valve and thereby opensaid bypass line; said first valve acting to open said vacuum line tothe atmosphere when closed, thereby allowing said biasing means to closesaid bypass valve and thereby close said bypass line.

12. A hydraulic drive system for a wheeled vehicle comprising: a firstfluid flow system for driving the front wheels and a second fluid flowsystem for driving the rear wheels of said vehicle, said first andsecond fluid flow systems each comprising at least one variabledisplacement pump and at least one variable displacement motor; meansinterconnecting said first and second fluid flow systems for equalizingpressure differentials which may arise therebetween during the operationof said vehicle; said means for equalizing pressure comprising apressure equalizing valve including a first chamber and a spool mountedtherein having two end portions, and a central portion around whichfluid in said first chamber cannot flow; said spool being mounted forslidable movement in said first chamber;

an opening into each end of said first chamber, being in flowcommunication with one of said two fluid flow systems; means biasingsaid spool towards a normal position in said first chamber; a secondchamber in said pressure equalizing valve; at least one passage betweensaid first and second chambers being positioned at each spool end whensaid spool is in its normal position, such that said passages arenormally blocked by said spool ends; said spool being slidable in saidfirst chamber such that upon increase in pressure in one of said fluidflow systems, said spool shifts away from said opening which is in flowcommunication with the increased pressure system, towards the otheropening in said first chamber such that said passages between said firstand second chambers are open; each end of said spool including passagesextending therethrough such that when said passages between said firstand second chambers are open, fluid can flow from the high pressure endof said first chamber, through said second chamber, back into said firstchamber in the space between said spool end and said spool centralportion and through said passages in said spool end and through saidopening in said first chamber into the lower pressure system.

13. The system of claim 12 in which said spool is biased by a springpositioned at each of said ends, said openings into each end of saidfirst chamber comprising a stand pipe extending into said chamber for a,

distance greater than the length of said spring when it is fullycompressed such that a seal is effected by the action of said spool endpressing against the end of said stand pipe in the event that one ofsaid two fluid flow systems completely depressurizes.

14. A hydraulic system for wheeled vehicles comprising: a pump; a motorfor at least one wheel being driven by said pump; a return reservoir anda bypass line to said reservoir whereby the fluid pumped by said pumpcan be returned through said bypass line to said reservoir rather thanbeing circulated to said motor; a bypass valve on said bypass lme foropening and closing said bypass line; a vacuum line to a vacuum source;first and second valve means positioned in series on said vacuum linewhereby said vacuum line is open only when both said first and secondvalve means are open; means for opening said first valve when saidengine accelerator is released and for closing said first valve whensaid engine accelerator is depressed; means for opening said secondvalve below a preselected engine speed and for closing said second valveabove said preselected engine speed; a control piston; means biasingsaid control piston to normally close said bypass valve; said controlpiston being connected to said vacuum line whereby the opening of saidfirst and second valves causes a vacuum to act on said control piston,against said biasing means, and thereby open I said bypass valve andthereby open said bypass line;

said first valve acting to open said vacuum line to the atmosphere whenclosed, thereby allowing said biasing means to close said bypass valveand thereby close said bypass line.

1. A hydraulic drive system for a wheeled vehicle comprising: a hydraulic pump having a means for varying its effective displacement; means for driving said hydraulic pump; a hydraulic motor for at least one wheel, said motor having means for varying its effective displacement and being driven by said pump; said means for varying the effective displacement of said pump including control means for controlling the effective displacement of said pump in direct proportion to the rate of rotational speed thereof; said means for varying the effective displacement of said motor including control means for controlling the effective displacement of said motor in inverse proportion to the rate of rotational speed thereof, whereby the torque to speed ratio at said driven wheel is greater at lower vehicle speeds than at higher vehicle speeds; each of said variable displacement pump and motor including a piston for controlling the displacement thereof, and each of said pump controlling means and said motor controlling means comprising: a valve having first and second chambers therein; first and second valve elements movably mounted in said first and second chambers respectively; a first opening in said first chamber being in flow communication with a source of fluid; a second opening in said second chamber being in flow communication with the displacement piston of one of said variable displacement pump and variable displacement motor; an intermediate opening between said first and second chambers, said valve elements being in slidable, abutting engagement at said intermediate opening; first and second spaced passages through said first valve element in flow communication with said first chamber opening and extending to said intermediate opening; a third passage in said second valve element in flow communication with said second chamber opening and extending to said intermediate opening; the opening of said third passage at said intermediate opening being positioned between the openings of said first and second passages at said intermediate opening whereby it is normally closed because of said slidable, abutting engagement of said first and second valve elements; means for increasing the pressure at one of said first chamber opening and second chamber opening with respect to the other and for moving one of said first and second passages into alignment with said third passage to effectuate fluid flow between said fluid source and said piston; means for moving said second valve element and correspondingly said third passage out of alignment with one of said first and second passages and in the same direction in which said first valve element was originally moved to thereby again discontinue the flow of fluid between said fluid source and said piston; said valve for said pump being adjusted such that fluid flows from said fluid source to said piston during vehicle deceleration and flows from said piston to said fluid source during vehicle acceleration; said valve for said motor being adjusted such that fluid flows from said piston to said fluid source during vehicle deceleration and from said fluid source to said piston during vehicle acceleration.
 2. The system of claim 2 in which said means for moving said second valve element in the same direction in which said first valve element was originally moved comprises said second valve chamber being in fluid flow communication with said second opening at one end of said second valve element, whereby changes in relative pressure at said second opening are gradually transmitted to said one end of said second valve element in said second chamber; bias means being provided at the other end of said second valve element for biasing it towarD one direction of movement within said second chamber.
 3. The system of claim 2 in which said means for moving said first valve element and for effectuating a pressure differential between said first and second chamber openings comprises: governor means for applying a force to said first valve element which is proportional to the rate of rotation of one of said pump and said motor; bias means being operably associated with said governor means for opposing said force applied to said governing means; pump means for said fluid source; bypass valve means associated with said fluid source and controlled by said governor for opening and closing a return line to said fluid source pump whereby the fluid pressure at said first chamber opening of said valve can be increased or decreased depending on the relative position of said bypass valve.
 4. A hydraulic drive system for a wheeled vehicle comprising: a hydraulic pump having a means for varying its effective displacement, means for driving said hydraulic pump; a hydraulic motor for at least one wheel, said motor having means for varying its effective displacement and being driven by said pump; said means for varying the effective displacement of said pump including control means for controlling the effective displacement of said pump in direct proportion to the rate of rotational speed thereof; said means for varying the effective displacement of said motor including control means for controlling the effective displacement of said motor in inverse proportion to the rate of rotational speed thereof, whereby the torque to speed ratio at said driven wheel is greater at lower vehicle speeds than at higher vehicle speeds; a first fluid flow system for driving the front wheels and a second fluid flow system for driving the rear wheels of said vehicle, said first and second fluid flow systems each comprising at least one variable displacement pump and at least one variable displacement motor; means interconnecting said first and second fluid flow systems for equalizing pressure differentials which may arise therebetween during the operation of said vehicle; said means for equalizing pressure comprising a pressure equalizing valve including a first chamber and a spool mounted therein having two end portions, and a central portion around which fluid in said first chamber cannot flow; said spool being mounted for slidable movement in said first chamber; an opening into each end of said first chamber, being in flow communication with one of said two fluid flow systems; means biasing said spool toward a normal position in said first chamber; a second chamber in said pressure equalizing valve; at least one passage between said first and second chambers being positioned at each spool end when said spool is in its normal position, such that said passages are normally blocked by said spool ends; said spool being slidable in said first chamber such that upon increase in pressure in one of said fluid flow systems, said spool shifts away from said opening which is in flow communication with the increased pressure system, toward the other opening in said first chamber such that said passages between said first and second chambers are open; each end of said spool including passages extending therethrough such that when said passages between said first and second chambers are open, fluid can flow from the high pressure end of said first chamber, through said second chamber, back into said first chamber in the space between said spool end and said spool central portion and through said passages in said spool end and through said opening in said first chamber into the lower pressure system.
 5. The system of claim 4 in which said spool is biased by a spring positioned at each of said ends, said openings into each end of said first chamber comprising a stand pipe extending into said chamber for a distance greater than the length of said spring when it is fully compressed such that a seal is effected by the action of said spool end pressing against the end of saiD stand pipe in the event that one of said two fluid flow systems completely depressurizes.
 6. A hydraulic drive system for a wheeled vehicle comprising: a hydraulic pump having a means for varying its effective displacement; means for driving said hydraulic pump; a hydraulic motor for at least one wheel, said motor having means for varying its effective displacement and being driven by said pump; said means for varying the effective displacement of said pump including control means for controlling the effective displacement of said pump in direct proportion to the rate of rotational speed thereof; said means for varying the effective displacement of said motor including control means for controlling the effective displacement of said motor in inverse proportion to the rate of rotational speed thereof, whereby the torque to speed ratio at said driven wheel is greater at lower vehicle speeds than at higher vehicle speeds; each said variable displacement pump and said variable displacement motor comprising: a vane track having a movable section whereby variable displacement is achieved; a rotor carrying a plurality of vanes mounted for radially slidable movement therein, a counterweight mounted in said rotor for each of said vanes and being operably connected to said vane for offsetting the centrifugal force acting on said vane during rotation of said rotor.
 7. The system of claim 6 in which said counterweight and said vane are connected through a pivoted arm; the moment arm of said vane about the pivot of said pivoted arm being slightly greater than the moment arm of said counterweight to held insure continuing contact between said vane and said track.
 8. The system of claim 7 in which said pivoted arm extends into a slot in said vane and fits loosely therein whereby said vane is free to move radially within said rotor while said pivoted arm moves through an arc about its pivot point.
 9. The system of claim 7 in which said counterweight and said vane are connected through a pivoted arm; bias means being operably connected to said arm for biasing said vane to movement radially outwardly whereby continuing contact between said vane and said track are insured.
 10. The system of claim 6 in which said counterweight and said vane are connected through a pivoted arm; bias means being operably connected to said arm for biasing said vane to movement radially outwardly whereby continuing contact between said vane and said track are insured.
 11. A hydraulic drive system for a wheeled vehicle comprising: a hydraulic pump having a means for varying its effective displacement; means for driving said hydraulic pump; a hydraulic motor for at least one wheel, said motor having means for varying its effective displacement and being driven by said pump; said means for varying the effective displacement of said pump including control means for controlling the effective displacement of said pump in direct proportion to the rate of rotational speed thereof; said means for varying the effective displacement of said motor including control means for controlling the effective displacement of said motor in inverse proportion to the rate of rotational speed thereof, whereby the torque to speed ratio at said driven wheel is greater at lower vehicle speeds than at higher vehicle speeds; a return reservoir and a bypass line to said reservoir are provided whereby the fluid pumped by said pump can be returned through said bypass line to said reservoir rather than being circulated to said motor; a bypass valve on said bypass line for opening and closing said bypass line; a vacuum line to a vacuum source; first and second valve means positioned in series on said vacuum line whereby said vacuum line is open only when both said first and second valve means are open; means for opening said first valve when said engine accelerator is released and for closing said first valve when said engine accelerator is depressed; means for opening said second valve below a preselected engine speed and for clOsing said second valve above said preselected engine speed; a control piston; means biasing said control piston to normally close said bypass valve; said control piston being connected to said vacuum line whereby the opening of said first and second valves causes a vacuum to act on said control piston, against said biasing means, and thereby open said bypass valve and thereby open said bypass line; said first valve acting to open said vacuum line to the atmosphere when closed, thereby allowing said biasing means to close said bypass valve and thereby close said bypass line.
 12. A hydraulic drive system for a wheeled vehicle comprising: a first fluid flow system for driving the front wheels and a second fluid flow system for driving the rear wheels of said vehicle, said first and second fluid flow systems each comprising at least one variable displacement pump and at least one variable displacement motor; means interconnecting said first and second fluid flow systems for equalizing pressure differentials which may arise therebetween during the operation of said vehicle; said means for equalizing pressure comprising a pressure equalizing valve including a first chamber and a spool mounted therein having two end portions, and a central portion around which fluid in said first chamber cannot flow; said spool being mounted for slidable movement in said first chamber; an opening into each end of said first chamber, being in flow communication with one of said two fluid flow systems; means biasing said spool towards a normal position in said first chamber; a second chamber in said pressure equalizing valve; at least one passage between said first and second chambers being positioned at each spool end when said spool is in its normal position, such that said passages are normally blocked by said spool ends; said spool being slidable in said first chamber such that upon increase in pressure in one of said fluid flow systems, said spool shifts away from said opening which is in flow communication with the increased pressure system, towards the other opening in said first chamber such that said passages between said first and second chambers are open; each end of said spool including passages extending therethrough such that when said passages between said first and second chambers are open, fluid can flow from the high pressure end of said first chamber, through said second chamber, back into said first chamber in the space between said spool end and said spool central portion and through said passages in said spool end and through said opening in said first chamber into the lower pressure system.
 13. The system of claim 12 in which said spool is biased by a spring positioned at each of said ends, said openings into each end of said first chamber comprising a stand pipe extending into said chamber for a distance greater than the length of said spring when it is fully compressed such that a seal is effected by the action of said spool end pressing against the end of said stand pipe in the event that one of said two fluid flow systems completely depressurizes.
 14. A hydraulic system for wheeled vehicles comprising: a pump; a motor for at least one wheel being driven by said pump; a return reservoir and a bypass line to said reservoir whereby the fluid pumped by said pump can be returned through said bypass line to said reservoir rather than being circulated to said motor; a bypass valve on said bypass line for opening and closing said bypass line; a vacuum line to a vacuum source; first and second valve means positioned in series on said vacuum line whereby said vacuum line is open only when both said first and second valve means are open; means for opening said first valve when said engine accelerator is released and for closing said first valve when said engine accelerator is depressed; means for opening said second valve below a preselected engine speed and for closing said second valve above said preselected engine speed; a control piston; means biasing said control piston to normally close said bypass valve; said control piston being connected to said vacuum line whereby the opening of said first and second valves causes a vacuum to act on said control piston, against said biasing means, and thereby open said bypass valve and thereby open said bypass line; said first valve acting to open said vacuum line to the atmosphere when closed, thereby allowing said biasing means to close said bypass valve and thereby close said bypass line. 